High capacity passive optical network

ABSTRACT

A passive optical network suitable for use as a telecommunications access network, having a head-end transmitting in a first optical frequency and one or more subscriber stations transmitting on a second common optical frequency. A carrier sense, multiple access protocol, with collision detection, is used, the collision detection being performed in the subscriber and head-end stations.

FIELD OF THE INVENTION

The present invention relates to apparatus and related methods ofoperation for providing a high capacity passive optical network, and asystem incorporating the same.

BACKGROUND OF THE INVENTION

This invention relates to fibre in the loop access networks, and inparticular to fibre to the home (FTTH).

A characteristic of known FTTH networks is that customers tend to existin groups situated geographically close to each other (say, within 200meters), but the head end (or central office) may be some kilometersaway for example, 5 km.

In networks where multiple nodes share a common medium, a multipleaccess protocol is needed to manage access to the medium such thatindividual nodes on the network can transmit their informationsuccessfully without interruption by transmission attempts from othernodes. Ethernet (IEEE standard 802.3) has adopted a ‘Carrier Sense’ with‘Collision Detection’ protocol. When a node has data to transmit, itmonitors the medium to check whether any other node is currentlytransmitting (Carrier Sense). If not, it is permitted to starttransmission. Other nodes on the network might also start transmittingwhen they detect that the medium is free, but might not know of othernearly simultaneous transmissions by another node because of theinherent transmission delay of the medium. Simultaneous transmissionresults in a ‘collision’ which must be detected by all nodessimultaneously attempting to transmit. Once a collision is detected,each node ceases transmission and waits for a random time intervalbefore sensing the medium and trying again.

Current implementations of Gigabit Optical Ethernet use point-to-pointoptical links to an ‘unbuffered repeater’ at the logical hub of thenetwork. The repeater demodulates incoming signals from thepoint-to-point links and identifies a collision when optical activityoccurs simultaneously on more than one input.

A disadvantage with this system is that it requires active electronicsin the repeater which is not compatible with operator requirements toremove active electronics from street locations.

It is also known to use a passive optical star coupler to interconnectnodes in an optical network. A star coupler has a number of inputs andoutputs where an optical signal on any input appears (attenuated) at allcoupler outputs. Various techniques are known for constructing passivestar couplers. One common technique is to interconnect a number ofsimpler fused fibre couplers, each having two inputs and two outputs, toform a larger matrix with the required functionality. For example, aneight by eight star coupler can be made up from 12 two by two couplers.

In a practical network, a coupler with an equal number of input andoutput ports, say eight input and eight output ports, would be used.Each node in the network connects to an input/output pair of ports. Inthis way, each node receives traffic sent by itself and all other nodeson the network. Carrier sense and collision detection can then beperformed by each node to implement the multiple access protocol.

For collision detection to work properly it is desirable (in the case ofGigabit Ethernet, mandatory) that any collision should be detectedbefore a transmitting node has finished the transmission. For high speednetworks such as these using a passive optical star coupler, thisresults in a tradeoff between the physical size of the network and theminimum length of the transmitted packet. For a network running at anominal bit of 1 Gbit/s and with an overall size of 5 km (typical of anaccess network) the minimum packet size to guarantee detection of acollision is around 6 kbytes.

A problem with this is that such a network is very inefficient for shortpackets, which must be extended to the minimum packet size to guaranteedetection of any possible collisions. In practice, a significantproportion of packets (such as those used for voice and TCP/IPacknowledgements) will be short.

A further problem is that, the maximum permitted packet size on anEthernet network is around 1500 bytes; any network including Ethernetsegments cannot therefore use a larger packet size than this unless datais reformatted.

Collision detection also requires that any transmitter must be able todetect reliably when more than one node is transmitting simultaneously.In a network interconnected with a passive optical coupler, tolerancesin the port to port coupler loss, differing attenuation in fibreconnections between nodes and the passive coupler, and variations innode transmitter output powers combine to give a wide variation inoptical signal level received at any node. It is then difficult todetect collisions, since a weak signal can be swamped by a strong one.Indeed, when practical tolerance levels are fully taken into account, itmay not be possible to build a suitable receiver economically.

An earlier version of Ethernet, running at 10 Mbit/s, included an optionto operate collision detection in the optical domain using a passivestar coupler (known in the standard as ‘10baseFP’). In practice, thissystem is rarely used because of the difficulty of implementing areceiver capable of reliable collision detection. The problemsassociated with implementing such a receiver can be expected to be muchmore severe at a bit rate of 1 Gbit/s.

OBJECT TO THE INVENTION

The invention seeks to provide improved apparatus and method ofoperation for providing a high capacity passive optical network, and asystem incorporating the same.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided apassive optical network arrangement comprising:

-   -   a head-end station;    -   at least one subscriber station;    -   a passive optical network providing optical connectivity from        each of said stations to each other station;        wherein said subscriber stations are arranged to transmit on a        common optical frequency distinct from that on which said        head-end station is arranged to transmit, and each of said        subscriber stations is arranged to detect when another of said        subscriber stations is transmitting over said passive optical        network.

Advantageously, upstream packet size in the arrangement is independentof the distance between the subscriber stations and the head station,though still dependent on the distance between subscriber stations. Thisallows the head-end to be situated distant from the subscriber stationswhilst still allowing upstream collision detection among theoutstations.

Advantageously, detection of collisions is performed by the subscriberstations rather than within the network connecting them, therebyavoiding the need for active collision detection within the connectingoptical network.

Advantageously, the components of the network connecting the stationsare passive, thereby obviating the need to provide power at any pointbetween the head-end station and the subscriber stations.

Advantageously, such a multiple access network allows fibre and head-endequipment to be shared across groups of end customers, resulting in amore cost effective infrastructure. A network requiring only passiveelements in outside locations would be more attractive, particularly toincumbent network operators who traditionally have not used activestreet equipment.

In a preferred embodiment, the subscriber station communicates with thehead-end station using a carrier sense/collision detection protocol.

Preferably the protocol is an Ethernet protocol.

Most preferably, the protocol operation at bit rates of the order of 1Gbit/s or above.

Advantageously, arrangements described are significantly more efficientthan known alternative solutions and would therefore be more attractiveto network operators.

Advantageously, the possibility of reusing existing technology designedfor optical Gigabit Ethernet offers the opportunity of achieving shortertime to market than developing entirely new technology for the system.

Advantageously, use of Gigabit Ethernet allows use of existinghigh-volume chips, thereby reducing system manufacturing costs.

In one preferred embodiment, the passive optical network providesoptical connectivity from each of said stations back to itself.

In such an arrangement, collision detection at a station involves acomparison between the newly sent signal and the received signal. Acollision has occurred if the received signal incorporates a componentarising from a signal transmitted by another station.

In a preferred embodiment, said passive optical network comprises;

-   -   a passive star coupler connected by means of point-to-point        optical links to each of the stations.

Advantageously, the amount of optical fibre requirements for the networkis minimised, and the interconnectivity between point-to-point links isprovided in a single location.

In a preferred embodiment, the passive optical network provides nooptical connectivity from each of said stations back to itself.

Advantageously, this allows considerable simplification of the collisiondetection apparatus in the outstations, since a collision isidentifiable if any signal is received on the shared transmissionfrequency during transmission by the outstation.

Advantageously, the collision domain of the protocol is restricted tothe subscriber stations, so that the constraint between protocol packetsize and maximum physical distance between stations applies only to thesubscriber stations—the head-end station is not restricted in that way.

Advantageously, this allows the head end to be at a considerably greaterdistance from each outstation compared with the distance betweenoutstations, thereby affording a greatly enhanced reach for the networkas a whole.

A telecommunications network comprising a passive optical networkarrangement according to the foregoing aspect of the present invention.

According to a further aspect of the present invention there is providedan optical transceiver arrangement comprising:

-   -   a transmitter arranged to transmit data on a first optical        frequency;    -   a transmission detector arranged to receive, on said first        optical frequency, signals from a network indicative of a        transmission by another subscriber station on said first        frequency;    -   a medium access logic unit arranged to prevent transmission on        said first frequency while said transmission detector is        detecting said signals from a network indicative of a        transmission by another subscriber station on said first        frequency.

In a preferred embodiment, the transceiver further comprises:

-   -   a receiver arranged to receive data on a second optical        frequency.

Advantageously, the transceiver may receive data on the second frequencyindependently of signals received on said first frequency.

In a preferred embodiment, the station comprises:

-   -   a common input port arranged to receive both said signal on said        first optical frequency and said signal on said second        frequency;    -   an optical frequency splitter arranged to provide said signal on        said first frequency to said transmission detector and said        signal on said second frequency to said receiver.

Advantageously, only a single input port is required, thereby reducingsubscriber equipment and network cost.

In a preferred embodiment, said indication comprises any non-zero signalon said first optical frequency.

Advantageously, this reduces the complexity required in the transmissiondetector circuitry. In particular no complex circuitry is required tocompare a recently transmitted signal with the received signal todetermine whether the received signal is or is not indicative of atransmission by another subscriber station

In a preferred embodiment, the transmission detector comprises a simplelight detector. Preferably, the light detector comprises a PIN diode.

Advantageously, this reduces size and cost of the subscriber stationequipment.

The invention is also directed to a method by which the describedapparatus operates and including method steps for carrying out everyfunction of the apparatus.

In particular there is provided a method of operating a passive opticalnetwork arrangement comprising:

-   -   a head-end station;    -   at least one subscriber station;    -   a passive optical network providing optical connectivity from        each of said stations to each other station;        comprising the steps of:    -   at least on of the subscriber station transmitting on an optical        frequency common to the subscriber stations and distinct from        that on which said head-end station is arranged to transmit;    -   at least one of the subscriber stations detecting when another        of said subscriber stations is transmitting on said common        optical frequency over said passive optical network.

There is also provided a method of operating an optical transceiverarrangement comprising:

-   -   transmitting data on a first optical frequency;    -   receiving, on said first optical frequency, signals from a        network indicative of a transmission by another subscriber        station on said first frequency;    -   preventing transmission on said first frequency while said        transmission detector is detecting said signals from a network        indicative of a transmission by another subscriber station on        said first frequency.

The invention is also directed to signals generated by the describedapparatus singly and in combination.

The preferred features may be combined as appropriate, as would beapparent to a skilled person, and may be combined with any of theaspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to show how the invention may be carried into effect,embodiments of the invention are now described below by way of exampleonly and with reference to the accompanying figures in which:

FIG. 1 shows a block diagram of a example of a system in accordance withthe present invention;

FIG. 2 shows a schematic diagram of a first example of a non-returnoptical coupler in accordance with the present invention;

FIG. 3 shows a schematic diagram of a second example of a non-returnoptical coupler in accordance with the present invention.

FIG. 4 shows an further embodiment of a star coupler arranged for3-fibre working;

FIG. 5 shows a still further embodiment of a star coupler arranged for2-fibre working;

FIG. 6 shows a block diagram of an example of a further system arrangedto make use of a non-return optical coupler.

DETAILED DESCRIPTION OF INVENTION

Referring now to FIG. 1, there is shown a CS/CD network suitable for useas an access network, and comprising a single head end 25 connecting toa number of subscriber outstations 21–23 via a passive optical starcoupler 20. Typically, the distance between any two outstations isassumed to be relatively small, perhaps 200 m, but the distance from thehead end to the coupler may be much greater, for example up to around 5km. The coupler may be located at a convenient point close to thesubscribers' outstations (for example in a street cabinet) and, being apassive arrangment, requires no electrical power supply.

Each outstation comprises an optical transmitter 211 transmitting onfrequency λ1 and controlled by a medium access logic unit 210, and anoptical receiver arrangement 212 arranged to receive signals on bothfrequency's λ1 and λ2.

The optical receiver arrangement may receive signals on both λ1 and λ2,and splits them by means of demux 2124, providing the data signal on λ2to a data receiver 2122, and signals indicative of collisions to a CS/CDreceiver 2123.

When a given outstation has information to transmit, the outstationCS/CD receiver 2123 first monitors the incoming fibre 271 for opticalactivity from other outstations. If no such activity is present, themedium access logic 210 allows the transmission to start.

During a transmission, the outstation CS/CD receiver 2123 monitors theincoming connection for signal activity indicative of a collision. If acollision is detected, the medium access logic 210 ceases transmissionand retries after a random time interval.

Upstream collisions are detected 252 at the head end by identifying anddiscarding incomplete or corrupted packets.

In the downstream direction, information transmitted from the head endis directed to all outstation data receivers 2122, each of which filtersthe packets based on addressing information in the packet header andpasses relevant packets to the higher layers of the outstation system.Downstream transmissions may take place concurrently with upstreamtransmissions since the downstream path uses a different wavelength λ2from the upstream wavelength λ1.

Various techniques are applicable to collision detection 2123, includingthe method specified in IEEE 802.3 for 10baseFP. As an alternative,which may be more straightforward to implement at the higher ratesrequired for an optical Gigabit Ethernet, collision detection could alsobe performed by subtracting from the received signal 27 the signal 272originating from the local transmitter 211.

The average optical power returned from the coupler as a result ofemissions from the local transmitter can be expected to vary dependingon the attenuation of the fibre path 27 between the outstation and thecoupler, the loss between the coupler ports in use and the output powerof the transmitting laser diode. However, for a given outstation, thisaverage power level will remain relatively constant over an extendedperiod of time. Likewise, the delay in the optical path between thetransmitter and receiver in a particular outstation will also remainrelatively constant over time. Each outstation can therefore predict theoptical power level received due to operation of the local transmitter,and its timing. Thus a delayed and suitably attenuated average of thetransmitted signal can be subtracted from the received signal in theelectrical domain. Any residue (above a noise floor level) thenindicates the presence of at least a second transmitted signal whichmust be the result of a collision. Successful transmissions, notinvolving a collision, can be used as measurement opportunities toreinforce or correct the local estimate of delay and returned signallevel.

Optionally, the head end 25 can be connected to the star coupler 20using a single optical fibre 26 (instead of a fibre pair) by addingwavelength multiplexers at each end of the fibre connection.

To increase the downstream capacity of the network, either initially oras an upgrade to an existing network, traffic in the downstreamdirection could use multiple wavelengths, each wavelength being detectedat one or more outstations using wavelength selective filters installedeither in the outstations or at the coupler site. In this way, anasymmetrical network is generated, having larger capacity in thedownstream direction.

Turning now to FIG. 2, there is shown a particularly preferredembodiment of a optical star coupler suitable for use in such an opticalnetwork.

The coupler has five receive ports 1R–5R and five corresponding transmitports 1T–5T. The ports are logically paired so that, for example input1R is paired with output 1T, 2R is paired with output 2T, etc. In manypossible applications of the coupler, it is anticipated that such a pairof ports would be coupled to an optical fibre, or pair of fibres,leading to a single network equipment.

The receive ports are connected to transmit ports by means of a latticearrangement of optical power splitters 100 and optical pathways 110 insuch a way as to convey signals received at any given receive port toeach transmit port other than the transmit port with which the receiveport is paired. Each splitter will have a theoretical power loss ofaround 3 dB. The nature of the construction gives rise to a number ofunused ports 120.

Where, as described above, a receive-transmit pair is coupled to singlenetwork equipment, this means that signals received from that equipmentare broadcast to all other connected equipments, but not back to theoriginating equipment.

Referring now to FIG. 3, there is shown a second star coupler, in thiscase supporting nine receive and transmit ports.

Preferred embodiments have 2″+1 port pairs, where n is an integergreater than or equal to 1, though for most envisaged applications, nwill be at least 2.

FIGS. 2 and 3 each show how two by two fused fibre couplers can bearranged to construct larger couplers having the property that anoptical signal on any input is directed to all output fibres, except tothe output fibre corresponding to the active input—a ‘non-return starcoupler’.

Since a node's own transmitted optical signal is not returned by thecoupler, any activity at the node's receiver can only result fromtransmissions from another node on the network. Thus, if any nodedetects optical activity whilst it is transmitting, then a collision hasoccurred. As a result, collision detection can be performed reliably bya much simpler receiver. For example, a simple light detector may beused comprising for example, a PIN diode.

Carrier sense works in the normal way.

Furthermore, since in such an arrangement only optical activity must bedetected, rather than a specific bit pattern at the network operatingbit rate, the optical receiver 2123 used for carrier sense and collisiondetection needs only a relatively low frequency response and cantherefore be implemented more economically.

The non-return coupler embodiments described above typically have anadditional 3 dB loss in excess of the intrinsic N-way splitting loss. Inan alternative embodiment shown in FIG. 4, this excess loss is reducedto less than 0.5 dB, through the use of WDM and asymmetric splitting.This arrangement requires 3-fibre working—1 fibre for upstream traffic,one for downstream traffic, and a third collision detect—to eachsubscriber station. The headend path does not require collisiondetection, since WDM is used to remove it from the collision detectdomain.

In this arrangement, the optimised coupler has a large asymmetric splitapplied to the upstream wavelength, directing the largest proportion(for example 90%) of the transmit power from an outstation towards theoutput fibre 5R directed towards the headend. As a result, the “ActivityDetect fibre outputs” 1AD–4AD will carry varying signal levels (derivedfrom 90% or 10% splits of the original outstation transmit levels)dependant upon which outstation is transmitting. This difference insignal levels is not a problem since the activity detect threshold willbe set to a value less than the minimum attenuated outstation signallevel expected.

All other couplers in the arrangement may be standard symmetrical 3 dBcouplers.

In a still further embodiment of a non-return star coupler, shown inFIG. 5, the fibre count on paths to the outstations is reduced to twofibres—one for transmit/receive, and one for Activity Detect. In PONarrangements using such star coupler, an additional WDM component isrequired in each outstation to separate the upstream and downstreamwavelengths.

Note that in Access Network Arrangements using either of these twoembodiments, the upstream direction can utilise either 2-fibre workingor single fibre working with WDM split at the headend, since each fibrecarries all upstream traffic.

Referring now to FIG. 6, there is shown a further embodiment of a CS/CDnetwork suitable for use as an access network, and comprising a numberof subscriber outstations 31–33 connectors via a non-return passiveoptical star coupler 30. Typically, the distance between any twooutstations is assumed to be relatively small, perhaps 200 m. Thecoupler may be located at a convenient point close to the subscribers'outstations (for example in a street cabinet) and, being a passivearrangment, requires no electrical power supply.

Each outstation comprises an optical transmitter 311 transmitting onfrequency λ1 and controlled by a medium access logic unit 310, and anoptical receiver arrangement 312 arranged to receive signals on the samefrequency, λ1.

The optical receiver arrangement is arranged to receive signals on λ1 ata data receiver 3122 and translates the received signals to theelectrical domain. Dat asignals are then forwarded to the local node forfurther processing whilst a copy is passed to the CS/CD receiver 3123.Any signal received in this arrangement is indicative of another stationtransmitting.

When a given outstation has information to transmit, the outstationCS/CD receiver 3123 first monitors the incoming fibre 371, via receiver3122, for optical activity from other outstations. If no such activityis present, the medium access logic 310 allows the transmission tostart.

During a transmission, the outstation CS/CD receiver 3123 monitors theincoming connection for signal activity indicative of a collision. If acollision is detected, the medium access logic 310 ceases transmissionand retries after a random time interval.

Upstream collisions are detected 52 at the head end by identifying anddiscarding incomplete or corrupted packets.

In the downstream direction, information transmitted from eachoutstation is directed to all outstation data receivers 3122, each ofwhich filters the packets based on addressing information in the packetheader and passes relevant packets on the higher layers of theoutstation system.

As in the embodiment of FIG. 1, since a node's own transmitted opticalsignal is not returned by the coupler, any activity at the node'sreceiver can only result from transmissions from another node on thenetwork. Thus, if any node detects optical activity whilst it istransmitting, then a collision has occurred. As a result, collisiondetection can be performed reliably by a much simpler receiver. Forexample, a simple light detector may be used comprising for example, aPIN diode.

Carrier sense works in the normal way.

Furthermore, since in such an arrangement only any signal activity mustbe detected, rather than a specific bit pattern at the network operatingbit rate, the receiver 2123 used for carrier sense and collisiondetection needs only a relatively low frequency response and cantherefore be implemented more economically.

In summary this invention allows, inter alia, an access network to bebuilt where the head end equipment and much of the fibre infrastructurecan be shared between a number of end customers. The transmissionefficiency is significantly increased by restricting the geographicalspan of the multiple access collision domain. This is achieved withoutrequiring active electronic equipment in street locations. Further, thepassive star coupler is expected to be highly reliable, so easy physicalaccess is also not required. The multiple access protocol can be similarto that used for Gigabit Ethernet, offering the possibility of reusingexisting technology.

Any range or device value given herein may be extended or alteredwithout losing the effect sought, as will be apparent to the skilledperson for an understanding of the teachings herein.

1. A passive optical network arrangement comprising: a head-end station;at least one subscriber station; a passive optical network providingoptical connectivity from each of said stations to each other station,but no optical connectivity from each of said stations back to itself;wherein said subscriber stations are arranged to transmit on a commonoptical frequency distinct from that on which said head-end station isarranged to transmit, and each of said subscriber stations is arrangedto detect when another of said subscriber stations is transmitting onsaid common optical frequency over said passive optical network.
 2. Apassive optical network arrangement according to claim 1 in which thesubscriber station communicates with the head-end station using acarrier sense/collision detection protocol.
 3. A passive optical networkarrangement according to claim 2 in which the protocol is an Ethernetprotocol.
 4. A passive optical network arrangement according to claim 2in which the protocol operates at bit rates of the order of 1 Gbit/s orabove.
 5. A passive optical network arrangement according to claim 1 inwhich said passive optical network comprises: a passive star couplerconnected by means of point-to-point optical links to each of thestations.
 6. A telecommunications access network comprising a passiveoptical network arrangement according to claim
 1. 7. Atelecommunications network comprising a passive optical networkarrangement according to claim
 1. 8. A passive optical networkarrangement according to claim 1 in which the passive optical networkcomprises a passive optical coupler comprising: a plurality of input andoutput port pairs; and arranged to couple each of said input ports tothe output port of each other input and output port pair.
 9. A passiveoptical network arrangement according to claim 8 in which the passiveoptical coupler comprises: a plurality of input ports each having acorresponding output port; wherein each input port is coupled to alloutput ports other than its corresponding output port.